FatCool schreef op 23 augustus 2020 09:00:
www.bmj.com/content/370/bmj.m3206Guidelines and governments must acknowledge the evidence and take steps to protect the public
In July, 239 scientists signed an open letter “appealing to the medical community and relevant national and international bodies to recognise the potential for airborne spread of covid-19.”1 Although the World Health Organization conceded that “airborne transmission cannot be ruled out,” the response was reserved and arguably mistaken in continuing to suggest that airborne and droplet transmission are discrete categories and that
airborne transmission occurs only during medical “aerosol generating procedures.”WHO defines droplets as =5-10 µm diameter and aerosols as <5 µm. However, both can be generated as a continuum of particle sizes during numerous respiratory activities and their behaviours are not distinct. This has important practical implications for infection control, the prevention of outbreaks and superspreading events, and for the new social behaviours that are being implemented in an effort to control the pandemic.
Aerosols are generated when the surface tension of fluid lining the respiratory tract is overcome by force.3 The required forces can be created by rapid shearing air flows, vocal cord movement, and the open and closing of terminal airways—all of which are influenced by the type and force of respiratory activity.3 Heavy breathing, coughing, talking, and singing all generate aerosols, causing an exhalation plume of respiratory particles of varying sizes, containing potentially infective viral material. The high viral loads present in the pharynx early in the course of covid-19 make these aerosols a plausible cause of both pre-symptomatic and asymptomatic transmission, which is so effective in fuelling outbreaks and yet difficult to control.
The arbitrary 5-10 µm threshold commonly used to dichotomise airborne and droplet transmission has never been supported theoretically or experimentally. Studies in both humans and airflow models show that particles as large as 50 µm can remain suspended and travel considerable distances.48 Furthermore, airborne range is influenced by the force and volume of exhalation as well as the local humidity, temperature, and airflow.
It is wrong to assume that droplets land only on exposed mucosal surfaces such as the eyes and mouth.4 Particles up to 50 µm can be captured by inspiratory airflows and are deposited along the much more extensive surface area of the respiratory tract; particles below 10 µm can penetrate as far as alveoli. The site of deposition may determine the viral dose required and severity of respiratory infection, as observed in influenza.